The ceramics sheet is widely utilized, for example, in a circuit substrate of electric parts, or separating plates for thin layer chromatography.
Alumina sheets used as ceramics circuit substrate have been prepared by the following steps of:
preparing a slurry by mixing powdery alumina with binders (e.g. organic binders, plasticizers, anticoagulants) and organic solvent;
applying the ceramics slurry on a carrier film in a specific thickness with a doctor blade;
drying the ceramics slurry developed on the carrier film by evaporating the organic solvent to give an alumina green sheet;
cutting the green sheet in a predetermined size and printing a circuit thereon;
laminating and pressing a plurarity of the processed sheets; and
drying and sintering the laminate to give a calcined alumina sheet (ceramics sheet) [Katsuyoshi Saito, "Moldings and Organic Materials of Fine Ceramics", pp 219-232, Published Aug. 26, 1985, C.M.C. Co., Japan].
The ceramics sheets obtained by the above procedures are used as a ceramics circuit substrate, and the above method is nowadays utilized for mass production.
Japanese Patent laid open No. 267157/1990 discloses a binder in an organic solvent, which comprises acrylic resin containing n-butyl methacrylate as a main component.
However, the above method and the binder encounter firstly with problems due to the organic solvent which needs the disaster prevention assessment and health control of operators, because the working circumstances become worse. Further, there is needed any assessment of pollution coming from outflow of the organic solvents (e.g. trichloroethylene, etc.) which would raise the cost of production. Accordingly, the above-mentioned method and the binder have defects such as complicated installations and troublesome operations.
Recently, a method of using water as a solvent (aqueous doctor blade method) has been investigated. In the aqueous doctor blade method is proposed a water-soluble binder. Japanese Patent laid open No. 226762/1989 discloses a binder for forming a ceramics sheet, which comprises a polymer obtained by neutralization of a polymer, which is produced by the reaction of an alkyleneimine with a copolymer having a carboxyl group, with acidic reagents. Japanese Patent laid open No. 236931/1989 teaches a dispersing agent containing a copolymer having a sulfonic acid group or its water-soluble salt, and Japanese Patent laid open No. 274831/1989 discloses a deflocculant containing a polyglycerol fatty acid ester as a main ingredient. In Japanese Patent laid open No. 286955/1989 is disclosed a latex obtained by emulsion or suspension polymerization of an oil-soluble monomer in the presence of a water-soluble (meth)acrylic resin. Furthermore, Japanese Patent laid open No. 290548/1989 discloses a ceramics slurry containing 500 to 800 parts by weight of water and 1 to 2 parts by weight of an organic solvent.
However, in the aqueous ceramics slurry containing these binders, powdery ceramics are apt to coagulate in the aqueous solvent, and the fluidity of ceramics slurry is lowered because of elevation of the viscosity, whereby powdery ceramics are insufficiently dispersed in the slurry. Therefore, the ceramics slurry causes a problem that a high density of a ceramics sheet having even surfaces is not obtained.
Since the surface tension of water is higher than that of organic solvents, an aqueous slurry also shows high surface tension. Therefore, when the ceramics slurry is applied on a carrier film by an aqueous doctor blade method, there still remains a problem that a ceramics green sheet cannot be formed in uniform thickness because the film repels the slurry.
Furthermore, when the ceramics green sheet thus obtained is dried, the problem of high shrinkage ratio of the sheet also remains unsolved.